Low-cost production of solar cells on flexible substrates using printing or web coating technologies is promising highly cost-efficient alternative to traditional silicon-based solar cells. Recently, solution-based solar cells fabricated from alloys of copper (Cu) and indium (In) with selenium (Se) or sulfur (S) have been developed. Such CIGS solar cells have been fabricated using a non-vacuum process in which a water-based precursor ink is formulated containing mixed oxides of Cu, In and Ga, which is then coated on rigid or flexible substrates (see U.S. Pat. No. 6,127,202, issued October 2000 to Kapur et. al., and U.S. Pat. No. 6,268,014 issued July 2001 to Eberspacher and Pauls). The resulting oxide mixture was then subject to reduction in H2/N2 mixture and selenization in an H2Se/N2 mixture at high temperatures between 400 to 500° C. The resulting CIGS solar cells typically have efficiency in the range of 8 to 11%. Another alternative ink-based approach used metallic powder paste to coat on substrates followed by selenization under H2Se/N2 at high temperature to form a CIS (or CIGS cell) solar cell (Kapur, V. K., et. al. Sol. Energy Mater Sol. Cells, 60 (2000) 127-134 and Kapur et al, Thin Solid Films, 431-432 (2003) 53-57 and also Kaelin, M., Meyer, T., Kurdesau, F., Rudmann, D., Zogg. H. and A. N. Tiawri. Low Cost Cu(In, Ga)Se2 Absorber Layers from Selenization of Precursor Materials, 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan, 2003). Unfortunately, the high temperature reduction and selenization steps used in each of these solar cell fabrication processes are neither cost effective nor easily scaled to high-volume production. Specifically, the H2Se gas used is both highly toxic and flammable. Thus, when H2Se is used at high temperatures, safety, environmental impact, and overall cost are serious concerns in the manufacturing process. Furthermore, the high temperature reduction and selenization steps make it impractical to make CIGS solar cells on inexpensive polymer or metallized polymer substrates that would warp, melt, or otherwise degrade at such high temperatures in the presence of reducing and/or oxidizing agents.
Although it is possible to make CIGS-based inks without using oxides, such inks have been produced in the prior art have several drawbacks for high-volume, roll-to-roll processing. For instance, the use of bulk CuInSe2 as a starting material is challenging as bulk CuInSe2 has a melting point around 1000° C. However, since most of the flexible substrates such as Al foils and plastic foils cannot withstand such a high temperature, it is not possible to melt bulk material directly onto a substrate. Even glass will have serious warping problems at this temperature and substrate warping typically leads to inefficient cell performance—so even with deposition onto glass, it is very difficult to create high-performance solar cells by melting bulk material. Moreover, the energy requirements needed for high temperature manufacturing at 1000° C. will incur substantial cost. Consequently, processes occurring at much lower temperatures are preferred. However, annealing at a lower temperature tends to hinder the manner of crystal grain growth that is critical for the proper electronic properties of CIGS solar cell. Certain fluxing agents have been used to reduce the melting point and sintering temperature for CuInSe2 (A. Vervaet et al. in 9th European Communities PV Solar Energy Conference, 1989, 480). Unfortunately, such fluxing agents can introduce unwanted crystalline phases and alter the electronic properties of CIGS, thus lowering the efficiency of a CIGS solar cell.
Thus, there is a need in the art, for a non-oxide based precursor ink that overcomes the above disadvantages.